31 October 2012

How Much Sea Level Rise Would be Avoided by Aggressive CO2 Reductions?

UPDATE 11/25: Here is another recent paper, by Schaeffer et al. in Nature Climate Change (PDF), with conclusions along the lines of the others summarized here: "mitigation measures, even an abrupt switch to zero emissions, have practically no effect on sea level over the coming 50 years and only a moderate effect on sea level by 2100."

UPDATE 11/2: At the Washington Post @bradplumer follows up this post with his own, providing a nice summary of the issue. He concludes: "aggressive steps to cut emissions could reduce the amount of sea-level
rise by somewhere between 6 and 20 inches in 2100, compared with our
current trajectory" -- which is just about exactly where I came out in the dicussions with him and several others (thanks JG), 10 inches +/- 10 inches.

UPDATE: Via Twitter @bradplumer points me to a newer paper that suggests perhaps 7 inches is the difference in seal level rise to 2100 between the highest and lowest RCP scenarios. It is not apples to apples with the number presented below, but still a very small number. And another paper here, with perhaps 10 inches between RCP scenarios, a number lower than the projection uncertainties.

One of the more reasonable discussion points to emerge from efforts to link Hurricane Sandy to the need to reduce carbon dioxide emissions focuses on the role that future sea level rise will have on making storm impacts worse. Logically, it would seem that if we can "halt the rise of the seas" then this would reduce future impacts from extreme events like Sandy.

The science of sea level rise, however, tells us that to 2100 (at least) our ability to halt he rise of the seas is extremely limited, even under an (unrealistically) aggressive scenario of emissions reduction. Several years ago, in a GRL paper titled "How much climate change can be avoided by mitigation?" Warren Washington and colleagues asked how much impact aggressive mitigation would have on the climate system. Specifically, they looked at a set of climate model runs assuming stabilization of carbon dioxide at 450 ppm.

Here is what they concluded for sea level rise:

[A]bout 8 cm of the sea level rise that would otherwise occur without mitigation would be averted. However, by the end of the century the sea level rise continues to increase and does not stabilize in both scenarios due to climate change commitment involving the thermal inertia of the oceans ...

Eight cm is about three inches. Three inches. Then sea level rise continues for centuries.

Though it seems logical to call for emissions reductions as a way to arrest sea level rise to reduce the impacts of hurricanes, recent research suggests that our ability to halt the rise of the seas is extremely limited. With respect to hurricanes, we have little option but to adapt, and improved adaptation makes good sense.

17 comments:

Not sure if this is in reference to the existence of hurricanes (or intensity) in itself, or w/r/t the impact of hurricanes - but if it is w/r/t impact, adaptation and mitigation are not mutually exclusive - as you seem to imply.

Given that there is some debate about the affect of emissions on intensity, and intensity affects not only sea levels, and that a fractional increase in sea levels can potentially have a disproportionate impact in damages, and that not all countries have the same capacity for adaptation - it seems to me that setting up an all or nothing paradigm in choosing between adaptation and mitigation is ill-advised.

The signal of GHG on hurricanes is not expected to be detectable until the latter part of the current century (and it may mean less storms overall and more of the intense variety). The signal in damage won't be detectable for 220 years +/-. (Emanuel says 120 yrs +/-.)

So when do you think we'd see the signal of GHG mitigation on storms? ;-) (Hint: Longer)

The relative effectiveness of mitigation vs. adaption is fairly simple to calculate here. It is not an argument against mitigation, just an argument against pretending that future hurricane impacts can be modulated in any meaningful way via energy policies.

It seems useful to emphasize that the reason mitigation has little effect on sea level in the Washington et al. study is simply because of the sea level rise we have committed to through the GHGs we have already emitted is slow to be realized; not because it is unrelated to GHGs. Thus any further emissions of GHGs leads to even greater sea level rise which will also be slowly realized over a century timescale. One could use this as a starting point for making a case for mitigation of the long lead time impacts of sea level rise while admitting that shorter time-scale impacts must mainly be adapted to.

"Three of the four climate models used produce increasing damage with time, with the global warming signal emerging on time scales of 40, 113, and 170 yr, respectively. It is pointed out, however, that probabilities of damage increase significantly well before such emergence time scales and it is shown that probability density distributions of aggregate damage become appreciably separated from those of the control climate on time scales as short as 25 yr. For the fourth climate model, damages decrease with time, but the signal is weak."

http://journals.ametsoc.org/doi/abs/10.1175/WCAS-D-11-00007.1

Why such a gap between emergence of a signal and signal of damage? As I suggested before, it seems to me that a fractional (small) increase in severity in just one storm - even if it doesn't reach a level of statistical significance when viewed in the context of all storms - can result in a disproportionate amount of damage.

Just as you are not arguing against mitigation, I am not arguing against adaptation (or "pretending" anything about the likely impact of mitigation short-term). I am arguing against setting up a mutually exclusive paradigm w/r/t adaptation vs. mitigation, as you seemed to be suggesting.

Roger: Meehl et al. only estimate sea level rise due to thermal expansion (steric sea level rise), and don't attempt to estimate the sea-level rise due to melting ice. Since most of the sea level rise in the next century is likely to be due to melting ice (something Meehl et al. explicitly state in their paper), your attempt to use this science to justify a policy recommendation is flawed.

Thanks, yes, I had a long conversation about this on Twitter yesterday with Brad Plumer and Michael Levi.

It is not clear from their paper how much, if any, of the ice sheet melting would be prevented by mitigation policies. The UCAR press release suggested very little. In any case, they did not quantify.

Fortunately, others have, which you can find linked in the update. In total, from Jevrejeva et al. at most about 10 inches of SLR might be avoided (actually difference between highest and lowest scenarios, not the same as "avoided" but let's go with it).

I think the uncertainties are far too large to specify a "top end" in the ball park of 10 inches for how much sea level rise would be prevented by aggressive mitigation.

When the range of estimates of 21st century sea level rise under BAU spans more than a meter, I don't believe that we understand it well enough to put a top-end of 10 inches on the impact of mitigation.

Has your trust in the accuracy of the predictions improved to the point where you think we can confidently estimate sea-level rise well enough that the uncertainty in the difference between two different predictions is less than 10 inches and then use that predicted difference to make policy decisions?

This sounds completely backward from what you said about predictions in your book with Sarewitz and Byerly.

Thanks, you are correct that the projected effects of mitigation are far smaller in the range of uncertainties in the estimates.

The paper does not provide predictions but rather a sensitivity analysis conditioned on a wide range of assumptions. ~10 inches is what comes out near the top of that range.

However, the policy recommendations that I make here are insensitive to that uncertainty.

I'm not clear what you are arguing here -- mitigation can prevent more sea level rise than projected in these studies? Mitigation is a meaningful tool in combating hurricane damage in the 21st century?

Roger: Two commments on the connection between the scientific studies you cite and your policy recommendation.

1. You emphasize the year-2100 impacts of mitigation, but there are substantial benefits of 20-century mitigation that will be realized after 2100. By 2150, the difference between the expectation value for the different RCP scenarios in the Jevrejeva paper linked at your update is about 1.5 meters.

2. The Jevrejeva paper indicates a 21-inch, not a 10-inch, difference between expectations for the RCP scenarios by 2100 (1.10 meters - 0.57 meters = 53 cm = 21 inches). And if you look at the upper-end of the 95% confidence interval, there's a 5% chance that aggressive mitigation would reduce sea-level rise by 32 inches by 2100 and by more than six feet by 2150 (I'm estimating from the graph on Figure 4(a)).

Reducing sea level rise by 6 feet is a lot different from reducing it by 3 inches or 10 inches, so I don't see where you come up with 10 inches as the greatest possible impact after conducting a sensitivity analysis.

1. Yes, I do emphasize 2100. If advocates for mitigation as a strategy to reduce surge losses would point to the fact that the main benefits are after 2100 I think that would be appropriate. As far as beyond 2100, how much of today's building stock dates to 1900 or before? Very little.

2. Jevrejeva at all start their analysis in 1990, which means that there are some inches already realized (maybe 3, 4? not sure). Also, comparing RCP 8.5 and 2.6 is not really the same as comparing a BAU projection with a stabilization scenario (e.g., 8.5 projects 1370 ppm eq to 2100 and a global population of 12B, 2.6 says emissions already peaked and are headed down -- neither is particularly realistic, and are not meant to be, but rather to bound assumptions).

I'll grant a comparison between 8.5 and 4.5 as a plausible representation of a BAU vs. a stabilization scenario -- hence ~10 inches, I'd be happy to add error bars of +/- 10 inches, which covers the ranges.

3. It is hard to see the picking of the most extreme scenarios and looking at the difference as anything other than cherry picking.

4. Please note that you seem to be arguing that I am opposed to mitigation. I am not (haven't we covered this before? ;-)

I am opposed to the suggestion that efforts to mitigate are a meaningful strategy of reducing storm surge damages to 2100. They are not, adaptation must be front and center.

But do let me know what proportion of future US hurricane damage to 2100 you think can be prevented via mitigation, by my math this comes up to be in the noise.

Roger: You started talking about comparing RCP's as a proxy for comparing a mitigation trajectory to stabilize CO2 at 450 to BAU.

RCP 2.6 is specifically defined as achieving radiative forcing of 450 ppm CO2e by 2100--the target you cited in your original post here. Why do you now call this "cherry picking?"

You say that RCP 8.5 is unrealistic, but its year-2100 emissions are less than what we'd get by following for a century Martin Hoffert's business-as-usual emissions trajectory (18 wedges worth of growth) that you endorsed here: http://rogerpielkejr.blogspot.com/2011/05/science-and-politics-of-wedges.html

So you used a more extreme emissions scenario than RCP 8.5 when you were criticizing Pacala and Socolow, but now you call RCP 8.5 an extreme cherry pick when I use it here.

Neither am I saying that you're opposed to mitigation---TCF is all about designing a policy that you think would be more effective at achieving mitigation than explicit emissions targets. I get that and I make damned sure that all my students are conversant with your arguments against GHG targets as policy goals.

Our past arguments have not been about whether or not to mitigate but generally about how important it is to cut emissions quickly (you don't feel as much urgency as I do, and that's a normative disagreement about risk aversion under uncertainty, not something that can be resolved rationally).

For this conversation, I started out replying to your "three inch" number for 450-ppm stabilization. Now we're arguing about your preference for 10 to 20 inches at year 2100 for holding GHGs to 630 ppm CO2e (i.e., RCP 4.5) versus my 21 to 34 inches for 475 ppm (RCP 2.6), so we're a lot closer together than when we started and most of the difference lies your switch from 450 to 630 ppm as the stabilization target.

1. Please note that the 3 inches is not "mine" -- it comes from Washington et al. and is part of the scientific literature.

2. The title of this post is "How Much Sea Level Rise Would be Avoided by Aggressive CO2 Reductions?"

To answer that question requires (a) an expectation for what SLR would be in the absence of CO2 reductions, and (b) an expectation for what SLR would be in the presence of CO2 reductions.

For (a) I've already granted you RCP 8.5 as a proxy for BAU (please no more silliness related to Hoffert, that discussion was to 2050 and not 2100), for (b) you would like to use RCP 2.6 which posits that emissions have already begun decreasing.

To answer the question posed in the title of this post, I'd prefer a less unrealistic scenario.

So according to Jevrejeva et al. 53 cm of SLR rise could be prevented from the assumptions of 8.5 to those of 2.6. Is that really the best answer to the question posed by this post? I don't think so, and, yes, reasonable people can disagree.

I read Jevrejeva et al. and see 36 cm as far more appropriate an answer (even they call it the most "realistic"), of which 6-7 cm is already in the books. To one sig digit, that is ~10 in, and if you'd like +/- 10 in to reflect the uncertainties fine with me.

So we have a different interpretation of the paper, probably not something to get worked up over. It is, after all, one study of several, and surely there will be more.

Please re-read the first sentence of this post. The answer to the question posed in the title is actually in relation to the discussions following Sandy.

Whether a 3 inch or 10 inch or 30 inch reduction in sea level rise is significant is a function of expected damage. When we are talking about 15, 20 or 30 foot storm surges a foot or two additional is not necessarily a significant factor in damage (fortunately, this can be quantified).

2. "you don't feel as much urgency as I do"

I disagree. The fastest way to cut emissions in a way that is practically possible (i.e., in the real world;-) are those outlined in The Hartwell Paper and TCF.

Exhortation is cathartic, but doesn't do much to move the decarbonization dial. Neither, I should add, are silly arguments about preventing future storm damage by changing energy policies.

In comment #12 you write to Roger, "1. You emphasize the year-2100 impacts of mitigation, but there are substantial benefits of 20-century mitigation that will be realized after 2100. By 2150,..."

Per Brad DeLong (World GDP, 1 million BC to present) the world GDP (in 1990 international dollars) was about $1.5 trillion in 1910. It is today about $50 trillion (in 1990 international dollars). So in the past 100 years, world GDP has increased by approximately a factor of approximately 33 (in international inflation-adjusted dollars).

What do you think the world GDP will be in 2112 (100 years from now) in 1990 international dollars? What about in 2150 (in 1990 international dollars)?